Portable electronic devices such as cellular telephones and laptop require a power adapter (typically a switching power converter) for recharging their batteries by convert converting the AC voltage and current from the AC mains into a regulated DC voltage or current for battery recharging. Due to the isolation provided by its transformer, a flyback switching power converter is typically selected as the power adapter for portable electronic devices. For safety and regulation, the flyback converter monitors the current it delivers to the load such as by including a secondary-side sense resistor in series with the transformer's secondary winding to measure the current delivered to the load. The sense resistance is typically quite low (e.g., 10 mΩ) to minimize its resistive power loss. At an output current of 5 A, such a sense resistor resistance would produce a 50 mV voltage drop across its terminals.
The voltage across the sense resistor during normal operation thus poses no risk to the sense resistor monitoring circuitry that measures the sense resistor voltage to determine the output current. However, a mishap or failure may lead to an open circuit at one of the sense resistor's terminals. In the case of an external sense resistor that is not integrated with the secondary-side controller for the flyback converter, the open circuit may result from a breakage of the solder used to mount the sense resistor to a printed circuit board or a break in the related trace of the printed circuit board. Modern lead-free solders that are designed to be more environmentally friendly tend to be more brittle than traditional lead-based solders and are thus more prone to such a failure. If the power converter tries to deliver current to the load with an open-circuited sense resistor, a much greater voltage can result across the sense resistor terminals than what the sense resistor monitoring circuit is designed for. In particular, the resulting high voltage can damage the internal circuits of the power converter controller, including its electrostatic discharge (ESD) circuits.
In addition to open-circuit-based faults, the sense resistor may also be short-circuited. In such a case there is no voltage across the terminals of the sense resistor with the resulting lack of voltage infoiining the controller that there is no current to the load. The controller may operate to supply more and more current to the load until damage occurs to the switching power converter, the load, or both.
Accordingly, there is a need in the art for a switching power converter controller that can detect both open-circuit faults and short-circuit faults for the sense resistor and that can adjust the operation of the switching power converter in response to the fault detection.